The most regretful thing I have ever done in my life is when the boy I liked said to me: "If you like me, then let's be together." I replied: "Who said you should be together if you like someone." If I had answered: "Okay!" Even if he was just joking or not serious, then I could have shamelessly been his girlfriend and occupied him in the past. Even a short period of time is enough for me to remember for the rest of my life.
It's just that I still haven't been able to realize that that boy is actually no longer mine.